The present invention pertains generally to interrupters in which a dielectric material in the liquid phase is used to quench an arc occurring during interruption, and pertains more specifically to interrupters of this type in which at least some of the power for the opening stroke of the interrupter is supplied by the arc itself.
It is well known that the use of sulfur hexafluoride (SF.sub.6) in circuit interrupters permits an interrupter of a given rating to be made considerably smaller than would otherwise be possible. The need for more efficient interrupters, capable of interrupting fault currents in excess of 40 kA and of recovering against the very steep rates of recovery voltages which are associated with the interruption of such currents, has made it desirable to be able to operate an SF.sub.6 interrupter at higher pressures than have hitherto been used. Because of the relatively low critical point of SF.sub.6, however, it has been necessary to limit the maximum operating pressure to a relatively low value, so that no liquefaction of the dielectric would occur even at the minimum operating temperature. To avoid this limitation, external heating elements have sometimes been used, but this expedient substantially increases the energy consumption of the circuit breaker accessories.
Interrupters in which the operating pressure is such as to maintain the SF.sub.6 in the liquid phase at all times, regardless of the ambient temperature, have also been devised. In one such interrupter, the dielectric liquid is caused to flow through the arcing region during interruption. This is done by providing two pistons rigidly connected to each other, with the contact assemblies located between them. A chamber communicating via an orifice with the arc region is provided in the interrupter between the stationary contact assembly and the adjacent piston. During interruption, the piston adjacent the stationary contact assembly moves toward the orifice, compressing the fluid in the chamber and forcing it through the orifice. At the same time, the other piston is moving away from the orifice and the contact assemblies, increasing the volume downstream of the orifice and thus producing a region of relatively low pressure. The resulting pressure differential causes the dielectric liquid to flow through the orifice and past the contact assemblies, quenching the arc as it does so. The orifice is designed to act as a nozzle, accelerating the liquid and increasing its quenching action. An interrupter of this type is disclosed in U.S. Pat. No. 4,268,733, issued May 19, 1981, to the present inventor for A LIQUID SF.sub.6 PUFFER TYPE CIRCUIT INTERRUPTER.
Since interrupting capability is primarily a function of the pressure differential across the nozzle, it will be understood that interruption of a high current requires a high pressure differential. However, the force required to develop a very high pressure differential in a quasi-incompressible fluid such as SF.sub.6 is very great, and this together with the length of time it must be sustained to ensure successful interruption of a current as large as 40 kA exceeds what can be achieved with operating mechanisms in use at the present time.
To avoid using excessively large and massive operating mechanisms, it is possible to utilize the energy liberated by the arc itself to power the interrupter. This energy can be used by making the effective area of the upstream and downstream pistons different, so that an increase in the fluid pressure in the region between them will exert a net accelerating force on the piston assembly in one direction (toward the piston having the larger effective area). An interrupter employing such a differential piston assembly is disclosed in U.S. Pat. No. 4,278,860, issued to Jiing-Liang Wu on July 14, 1981, for an ARC DRIVEN SINGLE PRESSURE TYPE CIRCUIT BREAKER.
The energy liberated by the arc is equal to the integral, taken over the total lifetime of the arc, of the product of the arc voltage and the arc current. It can be shown that this available arc energy is much greater than the energy available from any commercially available operating mechanism. Moreover, the arc energy, which is available free of charge, can be utilized to displace the pistons and create the pressure differential necessary to produce the dielectric liquid flow, by making use of the increase in enthalpy of the liquid SF.sub.6 that occurs as it absorbs energy from the arc. The increase in enthalpy results in an increase in pressure proportional to the current being interrupted. By making use of the arc energy and a suitably chosen difference between the two pistons' effective area, it is possible to produce quite large pressure differentials.
The use of a piston assembly comprising tandem-operated pistons having different effective areas, however, implies that at the end of the travel of the piston assembly the net volume occupied by the liquid SF.sub.6 in the space between the pistons is greater than at the beginning of the opening stroke, so that the final pressure in the interrupter is lower than before interruption. To repressurize the interrupter, it is necessary to do work on the liquid during the closing of the interrupter. One solution to this problem, disclosed in U.S. Pat. No. 4,278,860, is to store some of the energy produced by the arc. Either mechanical or pneumatic springs can be used for this purpose. The springs are compressed or charged while the pressure increases, and when the interruption has been completed and the pressure in the interrupter begins to fall, the springs supply the force necessary to return the piston assembly to the original equilibrium position. Although this approach is viable, it is somewhat cumbersome mechanically. In addition, interrupters of this type present certain manufacturing problems in that they require relatively large high pressure sliding seals between the pistons and the interrupter housing, and extremely fine finishes on the cylindrical walls of the pistons are required for acceptable sliding seal performance.